Methods and systems for optimizing sensor function by the application of voltage

ABSTRACT

A method is provided for initializing an analyte sensor, such as a glucose sensor. Where a sensor has been disconnected and reconnected, a disconnection time is determined and a sensor initialization protocol is selected based upon the disconnection time. The sensor initialization protocol may include applying a first series of voltage pulses to the sensor. A method for detecting hydration of a sensor is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Analyte sensors (e.g. glucose sensors used in the management ofdiabetes) and methods and materials for making and using such sensors.

2. Description of Related Art

Analyte sensors such as biosensors include devices that use biologicalelements to convert a chemical analyte in a matrix into a detectablesignal. There are many types of biosensors used for a wide variety ofanalytes. The most studied type of biosensor is the amperometric glucosesensor, which is crucial to the successful glucose level control fordiabetes.

A typical glucose sensor works according to the following chemicalreactions:

The glucose oxidase is used to catalyze the reaction between glucose andoxygen to yield gluconic acid and hydrogen peroxide (equation 1). TheH₂O₂ reacts electrochemically as shown in equation 2, and the currentcan be measured by a potentiostat. These reactions, which occur in avariety of oxidoreductases known in the art, are used in a number ofamperometric sensor designs.

As analyte sensor technology matures and new applications for sensortechnology are developed, there is a need for methods and materials thatfacilitate the use of sensors in new technological applications. Forexample, hospitals increasingly use continuous glucose sensors tomonitor patent physiology, for example in ICU environments. In suchhospital environments, situations arise where a sensor must bedisconnected from, and reconnected to, sensor electronics, for example,when a patient needs to undergo a magnetic resonance imaging (MRI)procedure. Because processors are incompatible with MRI, the sensorelectronics need to be disconnected from the sensor until the MRI iscompleted.

In conventional sensor setups, if a sensor is disconnected from and thenreconnected to sensor electronics, there is a significant delay beforethe sensor becomes stabilized enough to start sensing again. The delaycan last from several minutes to a couple of hours, thereby complicatingcare in clinical settings. In addition, in individuals using analytesensors in non-hospital settings (e.g. diabetics using glucose sensorsto manage their disease), relatively long sensor initialization and/orstart-up periods following sensor implantation can be problematical dueto both the inconvenience to the user as well as the delayed receipt ofinformation relating to user health. Because many diabetics do not havemedical training, they may forgo optimal monitoring and modulation ofblood glucose levels due to complexities associated with suchmanagement, for example, a two hour start-up period which can be aninconvenience in view of a patient's active daily routine.

For the above-noted reasons, methods and sensor systems that aredesigned to reduce sensor initialization and/or start-up times in aredesirable.

SUMMARY OF THE INVENTION

The invention disclosed herein provides methods and systems foroptimizing the initialization and performance of electrochemical analytesensors that have been temporarily disconnected from the electroniccomponents of their analyte monitoring systems. Embodiments of theinvention are useful, for example, in situations where an implantableelectrochemical glucose sensor is temporarily disconnected from theelectronic components of an analyte monitoring systems during a hospitalprocedure such as a magnetic resonance imaging (MRI) procedure.

The invention disclosed herein has a number of embodiments. Embodimentsof the invention include a method of initializing an electrochemicalanalyte sensor, the method comprising determining a disconnection time,wherein the disconnection time is the amount of time a sensor has beendisconnected from sensor electronics, and then selecting aninitialization protocol based on the disconnection time. In suchembodiments, the initialization protocol selected from the groupconsisting of: a first initialization scheme comprising applying a firstseries of voltage pulses to the sensor and a second initializationscheme comprising applying a second series of voltage pulses to thesensor, wherein the first initialization scheme is selected if thedisconnection time falls within a first time range and the secondinitialization scheme is selected if the disconnection time falls withina second time range. In such embodiments on can then apply the selectedinitialization protocol to the sensor. In some embodiments of theinvention, the initialization protocol is selected from the groupfurther consisting of a third initialization scheme comprising theapplication of no voltage to the sensor, wherein the thirdinitialization scheme is selected if the disconnection time is less thanthe first time range and the second time range.

Different embodiments of the invention can utilize differentdisconnection time ranges that, for example, can depend upon thespecific context in which a sensor is disconnected from sensorelectronics. In some embodiments of the invention, the first time rangecan be, for example, greater than 120 minutes (e.g. a range of 120minutes to at least 24 hours etc.). Similarly, in some embodiments ofthe invention, the second time range can be, for example, between 10 and120 minutes.

In embodiments, the method further comprises applying a stabilizationvoltage to the sensor, after applying the selected initializationvoltage, for a first stabilization time. The method may further includedetermining whether the sensor is stable after applying the firststabilization voltage; and if the sensor is not stable, applying asecond stabilization voltage to the sensor for a second stabilizationtime. Example stabilization time periods include times less than fortyminutes, such as 10, 16, 20, and 26 minutes, or 30 minutes. The secondstabilization time may be the same or different than the firststabilization time.

Some embodiments of the invention include calibrating the sensor afterthe stabilization of the sensor, if the sensor is stable. Calibrationmay include measuring blood glucose using a blood glucose meter andcorrelating the value found to the sensor measurements. In someembodiments, the calibration of the sensor is only performed if thedisconnection time falls within the first or second time range. In someembodiments of the invention, if the sensor is not stable after apredetermined maximum stabilization time, the initialization protocol isended so that a new sensor may be connected to the sensor electronics.The predetermined maximum stabilization period may be 30 minutes. Otherpredetermined maximum stabilization periods are also possible, such as35 or 40 minutes.

In embodiments, determining a disconnection time includes measuring thecurrent output of the sensor and comparing the measured output to adisconnection threshold value. One possible threshold value is 0.6 nA. Asample range of potential threshold values is 1-10 nA. In embodiments, atimer inbuilt in the program records time of disconnection. The event ofreconnection is detected when the current output is above a reconnectionthreshold, such as 4 nA.

In embodiments, the first initialization scheme includes the applicationof at least two voltages for a first predetermined initialization time.The at least two voltages may be pulsed, stepped or switched voltages.They may be applied in a repetitive sequence or each may be applied onlyonce. Similarly, the second initialization scheme includes theapplication of at least two voltages for a second predeterminedinitialization time. The at least two voltages may be pulsed, stepped orswitched voltages. They may be applied in a repetitive sequence or eachmay be applied only once. The predetermined second initialization timemay be less than 30 minutes.

Some embodiments of the invention include detecting hydration of thesensor prior to applying the selected initiation protocol, whereindetecting hydration includes applying a series of hydration pulses tothe sensor for a first hydration time; recording the current response ofthe sensor during application of the series of hydration pulses; andcomparing the current response to a predetermined hydration threshold.Application of the series of hydration pulses may be terminated if thecurrent response reaches or exceeds the predetermined hydrationthreshold. Detecting hydration may further include applying a secondseries of hydration pulses to the sensor for a second hydration time ifthe current response does not reach the predetermined hydrationthreshold during the first predetermined hydration time. Thepredetermined hydration threshold may be 100 nA or 50 nA, for example.Example hydration pulses may be a series of 0 V and 2 V pulses, forexample for 20 seconds or 2 minutes each.

In some embodiments of the invention, the analyte sensing system furthercomprises a monitoring device in communication with the electronicsdevice, wherein the monitoring device includes circuitry to monitor thesignals received from the analyte sensor and to calculate theconcentration of the analyte from the signals. The monitoring device maybe connected directly to the sensor and/or sensor electronics or mayreceive data wirelessly. The sensor electronics may be part of themonitor or separate from the monitor.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription. It is to be understood, however, that the detaileddescription and specific examples, while indicating some embodiments ofthe present invention are given by way of illustration and notlimitation. Many changes and modifications within the scope of thepresent invention may be made without departing from the spirit thereof,and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a schematic of the well-known reaction between glucoseand glucose oxidase. As shown in a stepwise manner, this reactioninvolves glucose oxidase (GOx), glucose and oxygen in water. In thereductive half of the reaction, two protons and electrons aretransferred from 1-D-glucose to the enzyme yielding d-gluconolactone. Inthe oxidative half of the reaction, the enzyme is oxidized by molecularoxygen yielding hydrogen peroxide. The d-gluconolactone then reacts withwater to hydrolyze the lactone ring and produce gluconic acid. Incertain electrochemical sensors of the invention, the hydrogen peroxideproduced by this reaction is oxidized at the working electrode(H₂O₂→2H⁺+O₂+2e⁻).

FIG. 2 provides a diagrammatic view of a typical layered analyte sensorconfiguration of the current invention.

FIG. 3 provides a perspective view illustrating a subcutaneous sensorinsertion set, a telemetered characteristic monitor transmitter device,and a data receiving device embodying features of the invention.

FIG. 4 provides a flow chart illustrating selection of an initializationscheme according to an embodiment of the invention.

FIG. 5 provides a flow chart illustrating an initialization schemeaccording to an embodiment of the invention.

FIG. 6 provides a flow chart illustrating an initialization schemeaccording to an embodiment of the invention.

FIG. 7 provides a flow chart illustrating an initialization schemeaccording to an embodiment of the invention.

FIG. 8A provides a graph showing the potential (V) versus time (s)according to one embodiment of the present invention.

FIG. 8B provides a graph showing the potential (V) versus time (s)according to one embodiment of the present invention.

FIG. 9A provides a graph showing the signal response (iSig) when asensor is reconnected to a processor after 2 hours of disconnection.

FIG. 9B provides a graph showing the signal response (iSig) when asensor is reconnected to a processor after 2 hours of disconnection andinitialized according to an embodiment of the present invention.

FIG. 10 provides a graphical illustration of an initialization schemeaccording to an embodiment of the invention

FIG. 11 provides a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 12 provides a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 13 is a graphical illustration of a hydration scheme according toan embodiment of the invention.

FIG. 14 is a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 15 is a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 16 is a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 17 is a graphical illustration of an initialization schemeaccording to an embodiment of the invention.

FIG. 18 is a graph showing a boxplot of recovery time for certaininitialization schemes according to the present invention.

FIG. 19 is a graph showing a boxplot of recovery time based on theamount of time in an initialization scheme of the present invention.

FIG. 20 shows a schematic of a potentiostat that may be used to measurecurrent according to embodiments of the present invention. As shown inFIG. 20, a potentiostat 300 may include an op amp 310 that is connectedin an electrical circuit so as to have two inputs: Vset and Vmeasured.As shown, Vmeasured is the measured value of the voltage between areference electrode and a working electrode. Vset, on the other hand, isthe optimally desired voltage across the working and referenceelectrodes. The current between the counter and reference electrode ismeasured, creating a current measurement (isig) that is output from thepotentiostat.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Unless otherwise defined, all terms of art, notations and otherscientific terms or terminology used herein are intended to have themeanings commonly understood by those of skill in the art to which thisinvention pertains. In some cases, terms with commonly understoodmeanings are defined herein for clarity and/or for ready reference, andthe inclusion of such definitions herein should not necessarily beconstrued to represent a substantial difference over what is generallyunderstood in the art. Many of the techniques and procedures describedor referenced herein are well understood and commonly employed usingconventional methodology by those skilled in the art. As appropriate,procedures involving the use of commercially available kits and reagentsare generally carried out in accordance with manufacturer definedprotocols and/or parameters unless otherwise noted. A number of termsare defined below.

All publications mentioned herein are incorporated herein by referenceto disclose and describe the methods and/or materials in connection withwhich the publications are cited. Publications cited herein are citedfor their disclosure prior to the filing date of the presentapplication. Nothing here is to be construed as an admission that theinventors are not entitled to antedate the publications by virtue of anearlier priority date or prior date of invention. Further the actualpublication dates may be different from those shown and requireindependent verification.

It is to be understood that this invention is not limited to theparticular methodology, protocol and reagent described as such may, ofcourse, vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to limit the scope of the present invention which willbe limited only by the appended claims.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “and”, and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “anoxidoreductase” includes a plurality of such oxidoreductases andequivalents thereof known to those skilled in the art, and so forth. Allnumbers recited in the specification and associated claims that refer tovalues that can be numerically characterized with a value other than awhole number (e.g. the concentration of a compound in a solution) areunderstood to be modified by the term “about”.

The term “analyte” as used herein is a broad term and is used in itsordinary sense, including, without limitation, to refer to a substanceor chemical constituent in a fluid such as a biological fluid (forexample, blood, interstitial fluid, cerebral spinal fluid, lymph fluidor urine) that can be analyzed. Analytes can include naturally occurringsubstances, artificial substances, metabolites, and/or reactionproducts. In some embodiments, the analyte for measurement by thesensing regions, devices, and methods is glucose. However, otheranalytes are contemplated as well, including but not limited to,lactate. Salts, sugars, proteins fats, vitamins and hormones naturallyoccurring in blood or interstitial fluids can constitute analytes incertain embodiments. The analyte can be naturally present in thebiological fluid or endogenous; for example, a metabolic product, ahormone, an antigen, an antibody, and the like. Alternatively, theanalyte can be introduced into the body or exogenous, for example, acontrast agent for imaging, a radioisotope, a chemical agent, afluorocarbon-based synthetic blood, or a drug or pharmaceuticalcomposition, including but not limited to insulin. The metabolicproducts of drugs and pharmaceutical compositions are also contemplatedanalytes.

The term “sensor,” as used herein, is a broad term and is used in itsordinary sense, including, without limitation, the portion or portionsof an analyte-monitoring device that detects an analyte. In oneembodiment, the sensor includes an electrochemical cell that has aworking electrode, a reference electrode, and optionally a counterelectrode passing through and secured within the sensor body forming anelectrochemically reactive surface at one location on the body, anelectronic connection at another location on the body, and a membranesystem affixed to the body and covering the electrochemically reactivesurface. During general operation of the sensor, a biological sample(for example, blood or interstitial fluid), or a portion thereof,contacts (directly or after passage through one or more membranes ordomains) an enzyme (for example, glucose oxidase); the reaction of thebiological sample (or portion thereof results in the formation ofreaction products that allow a determination of the analyte level in thebiological sample.

The terms “electrical potential” and “potential” as used herein, arebroad terms and are used in their ordinary sense, including, withoutlimitation, the electrical potential difference between two points in acircuit which is the cause of the flow of a current. The term “systemnoise,” as used herein, is a broad term and is used in its ordinarysense, including, without limitation, unwanted electronic ordiffusion-related noise which can include Gaussian, motion-related,flicker, kinetic, or other white noise, for example.

As discussed in detail below, embodiments of the invention relate to theuse of an electrochemical sensor that measures a concentration of ananalyte of interest or a substance indicative of the concentration orpresence of the analyte in fluid. In some embodiments, the sensor is acontinuous device, for example a subcutaneous, transdermal, orintravascular device. In some embodiments, the device can analyze aplurality of intermittent blood samples. The sensor embodimentsdisclosed herein can use any known method, including invasive, minimallyinvasive, and non-invasive sensing techniques, to provide an outputsignal indicative of the concentration of the analyte of interest.Typically, the sensor is of the type that senses a product or reactantof an enzymatic reaction between an analyte and an enzyme in thepresence of oxygen as a measure of the analyte in vivo or in vitro. Suchsensors typically comprise a membrane surrounding the enzyme throughwhich an analyte migrates. The product is then measured usingelectrochemical methods and thus the output of an electrode systemfunctions as a measure of the analyte.

Embodiments of the invention disclosed herein provide sensors of thetype used, for example, in subcutaneous or transcutaneous monitoring ofblood glucose levels in a diabetic patient. A variety of implantable,electrochemical biosensors have been developed for the treatment ofdiabetes and other life-threatening diseases. Many existing sensordesigns use some form of immobilized enzyme to achieve theirbio-specificity. Embodiments of the invention described herein can beadapted and implemented with a wide variety of known electrochemicalsensors, including for example, U.S. Patent Application No. 20050115832,U.S. Pat. Nos. 6,001,067, 6,702,857, 6,212,416, 6,119,028, 6,400,974,6,595,919, 6,141,573, 6,122,536, 6,512,939 5,605,152, 4,431,004,4,703,756, 6,514,718, 5,985,129, 5,390,691, 5,391, 250, 5,482,473,5,299,571, 5,568,806, 5,494,562, 6,120,676, 6,542,765, 7,033,336 as wellas PCT International Publication Numbers WO 01/58348, WO 04/021877, WO03/034902, WO 03/035117, WO 03/035891, WO 03/023388, WO 03/022128, WO03/022352, WO 03/023708, WO 03/036255, WO03/036310 WO 08/042,625, and WO03/074107, and European Patent Application EP 1153571, the contents ofeach of which are incorporated herein by reference.

While some embodiments of the invention pertain to glucose and/orlactate sensors, the methods disclosed herein can be adapted for usewith any one of the wide variety of sensors known in the art. Theanalyte sensor elements, architectures and methods for making and usingthese elements that are disclosed herein can be used to establish avariety of layered sensor structures. Such sensors of the inventionexhibit a surprising degree of flexibility and versatility,characteristics which allow a wide variety of sensor configurations tobe designed to examine a wide variety of analyte species.

In typical embodiments of the present invention, the transduction of theanalyte concentration into a processable signal is by electrochemicalmeans. These transducers may include any of a wide variety ofamperometric, potentiometric, or conductimetric base sensors known inthe art. Moreover, the microfabrication sensor techniques and materialsof the instant invention may be applied to other types of transducers(e.g., acoustic wave sensing devices, thermistors, gas-sensingelectrodes, field-effect transistors, optical and evanescent field waveguides, and the like) fabricated in a substantially nonplanar, oralternatively, a substantially planar manner. A useful discussion andtabulation of transducers which may be exploited in a biosensor as wellas the kinds of analytical applications in which each type of transduceror biosensor, in general, may be utilized, is found in an article byChristopher R. Lowe in Trends in Biotech. 1984, 2(3), 59-65.

Specific aspects of embodiments of the invention are discussed in detailin the following sections.

I. Typical Elements, Configurations and Analyte Sensor Embodiments ofthe Invention A. Typical Architectures Found in of Embodiments of theInvention

FIG. 2 illustrates a cross-section of a typical sensor embodiment 100 ofthe present invention. This sensor embodiment is formed from a pluralityof components that are typically in the form of layers of variousconductive and non-conductive constituents disposed on each otheraccording to art accepted methods and/or the specific methods of theinvention disclosed herein. The components of the sensor are typicallycharacterized herein as layers because, for example, it allows for afacile characterization of the sensor structure shown in FIG. 2.Artisans will understand however, that in certain embodiments of theinvention, the sensor constituents are combined such that multipleconstituents form one or more heterogeneous layers. In this context,those of skill in the art understand that the ordering of the layeredconstituents can be altered in various embodiments of the invention.

The embodiment shown in FIG. 2 includes a base layer 102 to support thesensor 100. The base layer 102 can be made of a material such as a metaland/or a ceramic and/or a polymeric substrate, which may beself-supporting or further supported by another material as is known inthe art. Embodiments of the invention include a conductive layer 104which is disposed on and/or combined with the base layer 102. Typicallythe conductive layer 104 comprises one or more electrodes. An operatingsensor 100 typically includes a plurality of electrodes such as aworking electrode, a counter electrode and a reference electrode. Otherembodiments may also include a plurality of working and/or counterand/or reference electrodes and/or one or more electrodes that performsmultiple functions, for example one that functions as both as areference and a counter electrode.

As discussed in detail below, the base layer 102 and/or conductive layer104 can be generated using many known techniques and materials. Incertain embodiments of the invention, the electrical circuit of thesensor is defined by etching the disposed conductive layer 104 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 100 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating cover layer106 such as a polymer coating can be disposed on portions of the sensor100. Acceptable polymer coatings for use as the insulating protectivecover layer 106 can include, but are not limited to, non-toxicbiocompatible polymers such as silicone compounds, polyimides,biocompatible solder masks, epoxy acrylate copolymers, or the like. Inthe sensors of the present invention, one or more exposed regions orapertures 108 can be made through the cover layer 106 to open theconductive layer 104 to the external environment and to, for example,allow an analyte such as glucose to permeate the layers of the sensorand be sensed by the sensing elements. Apertures 108 can be formed by anumber of techniques, including laser ablation, tape masking, chemicalmilling or etching or photolithographic development or the like. Incertain embodiments of the invention, during manufacture, a secondaryphotoresist can also be applied to the protective layer 106 to definethe regions of the protective layer to be removed to form theaperture(s) 108. The exposed electrodes and/or contact pads can alsoundergo secondary processing (e.g. through the apertures 108), such asadditional plating processing, to prepare the surfaces and/or strengthenthe conductive regions.

In the sensor configuration shown in FIG. 2, an analyte sensing layer110 (which is typically a sensor chemistry layer, meaning that materialsin this layer undergo a chemical reaction to produce a signal that canbe sensed by the conductive layer) is disposed on one or more of theexposed electrodes of the conductive layer 104. Typically, the analytesensing layer 110 is an enzyme layer. Most typically, the analytesensing layer 110 comprises an enzyme capable of producing and/orutilizing oxygen and/or hydrogen peroxide, for example the enzymeglucose oxidase. Optionally the enzyme in the analyte sensing layer iscombined with a second carrier protein such as human serum albumin,bovine serum albumin or the like. In an illustrative embodiment, anoxidoreductase enzyme such as glucose oxidase in the analyte sensinglayer 110 reacts with glucose to produce hydrogen peroxide, a compoundwhich then modulates a current at an electrode. As this modulation ofcurrent depends on the concentration of hydrogen peroxide, and theconcentration of hydrogen peroxide correlates to the concentration ofglucose, the concentration of glucose can be determined by monitoringthis modulation in the current. In a specific embodiment of theinvention, the hydrogen peroxide is oxidized at a working electrodewhich is an anode (also termed herein the anodic working electrode),with the resulting current being proportional to the hydrogen peroxideconcentration. Such modulations in the current caused by changinghydrogen peroxide concentrations can by monitored by any one of avariety of sensor detector apparatuses such as a universal sensoramperometric biosensor detector or one of the other variety of similardevices known in the art such as glucose monitoring devices produced byMedtronic Diabetes.

In embodiments of the invention, the analyte sensing layer 110 can beapplied over portions of the conductive layer or over the entire regionof the conductive layer. Typically the analyte sensing layer 110 isdisposed on the working electrode which can be the anode or the cathode.Optionally, the analyte sensing layer 110 is also disposed on a counterand/or reference electrode. While the analyte sensing layer 110 can beup to about 1000 microns (μm) in thickness, typically the analytesensing layer is relatively thin as compared to those found in sensorspreviously described in the art, and is for example, typically less than1, 0.5, 0.25 or 0.1 microns in thickness. As discussed in detail below,some methods for generating a thin analyte sensing layer 110 includebrushing the layer onto a substrate (e.g. the reactive surface of aplatinum black electrode), as well as spin coating processes, dip anddry processes, low shear spraying processes, ink jet printing processes,silk screen processes and the like. In certain embodiments of theinvention, brushing is used to: (1) allow for a precise localization ofthe layer; and (2) push the layer deep into the architecture of thereactive surface of an electrode (e.g. platinum black produced by anelectrodeposition process).

Typically, the analyte sensing layer 110 is coated and or disposed nextto one or more additional layers. Optionally, the one or more additionallayers includes a protein layer 116 disposed upon the analyte sensinglayer 110. Typically, the protein layer 116 comprises a protein such ashuman serum albumin, bovine serum albumin or the like. Typically, theprotein layer 116 comprises human serum albumin. In some embodiments ofthe invention, an additional layer includes an analyte modulating layer112 that is disposed above the analyte sensing layer 110 to regulateanalyte contact with the analyte sensing layer 110. For example, theanalyte modulating membrane layer 112 can comprise a glucose limitingmembrane, which regulates the amount of glucose that contacts an enzymesuch as glucose oxidase that is present in the analyte sensing layer.Such glucose limiting membranes can be made from a wide variety ofmaterials known to be suitable for such purposes, e.g., siliconecompounds such as polydimethyl siloxanes, polyurethanes, polyureacellulose acetates, Nafion, polyester sulfonic acids (e.g. Kodak AQ),hydrogels or any other suitable hydrophilic membranes known to thoseskilled in the art.

In some embodiments of the invention, the architecture or thickness of asensor layer is used to optimize a property of the sensor. For examplein some embodiments of the invention, the elongated base layer iscomprised of a dielectric or polyimide ceramic material that is at least100 microns thick. In some embodiments of the invention, the analytemodulating layer is at least 6, 7, 8, 9, 10, 15, 20, 25 or 30 micronsthick. Certain embodiments of the invention use a thick layer (e.g. 25or 30 microns) of an analyte modulating layer because in suchembodiments, this thick layer is observed to both optimize the linearityof an analyte signal over a range of signals (e.g. glucoseconcentration). Such thick layers have further properties that aredesirable in certain embodiments of the invention, for example a longeranalyte modulating layer lifetime (e.g. due to the extra material), aproperty that makes them particularly suited for certain long termsensor embodiments.

In typical embodiments of the invention, an adhesion promoter layer 114is disposed between the analyte modulating layer 112 and the analytesensing layer 110 as shown in FIG. 2 in order to facilitate theircontact and/or adhesion. In a specific embodiment of the invention, anadhesion promoter layer 114 is disposed between the analyte modulatinglayer 112 and the protein layer 116 as shown in FIG. 2 in order tofacilitate their contact and/or adhesion. The adhesion promoter layer114 can be made from any one of a wide variety of materials known in theart to facilitate the bonding between such layers. Typically, theadhesion promoter layer 114 comprises a silane compound. In alternativeembodiments, protein or like molecules in the analyte sensing layer 110can be sufficiently crosslinked or otherwise prepared to allow theanalyte modulating membrane layer 112 to be disposed in direct contactwith the analyte sensing layer 110 in the absence of an adhesionpromoter layer 114.

In certain embodiments of the invention, a sensor is designed to includeadditional layers such as an interference rejection layer discussedbelow.

B. Typical Analyte Sensor Constituents Used in Embodiments of theInvention

The following disclosure provides examples of typicalelements/constituents used in sensor embodiments of the invention. Whilethese elements can be described as discreet units (e.g. layers), thoseof skill in the art understand that sensors can be designed to containelements having a combination of some or all of the material propertiesand/or functions of the elements/constituents discussed below (e.g. anelement that serves both as a supporting base constituent and/or aconductive constituent and/or a matrix for the analyte sensingconstituent and which further functions as an electrode in the sensor).Those in the art understand that these thin film analyte sensors can beadapted for use in a number of sensor systems such as those describedbelow.

Base Constituent

Sensors of the invention typically include a base constituent (see, e.g.element 102 in FIG. 2). The term “base constituent” is used hereinaccording to art accepted terminology and refers to the constituent inthe apparatus that typically provides a supporting matrix for theplurality of constituents that are stacked on top of one another andcomprise the functioning sensor. In one form, the base constituentcomprises a thin film sheet of insulative (e.g. electrically insulativeand/or water impermeable) material. This base constituent can be made ofa wide variety of materials having desirable qualities such asdielectric properties, water impermeability and hermeticity. Somematerials include metallic, and/or ceramic and/or polymeric substratesor the like.

The base constituent may be self-supporting or further supported byanother material as is known in the art. In one embodiment of the sensorconfiguration shown in FIG. 2, the base constituent 102 comprises aceramic. Alternatively, the base constituent comprises a polymericmaterial such as a polyimide. In an illustrative embodiment, the ceramicbase comprises a composition that is predominantly Al₂O₃ (e.g. 96%). Theuse of alumina as an insulating base constituent for use withimplantable devices is disclosed in U.S. Pat. Nos. 4,940,858, 4,678,868and 6,472,122 which are incorporated herein by reference. The baseconstituents of the invention can further include other elements knownin the art, for example hermetical vias (see, e.g. WO 03/023388).Depending upon the specific sensor design, the base constituent can berelatively thick constituent (e.g. thicker than 50, 100, 200, 300, 400,500 or 1000 microns). Alternatively, one can utilize a nonconductiveceramic, such as alumina, in thin constituents, e.g., less than about 30microns.

Conductive Constituent

The electrochemical sensors of the invention typically include aconductive constituent disposed upon the base constituent that includesat least one electrode for contacting an analyte or its byproduct (e.g.oxygen and/or hydrogen peroxide) to be assayed (see, e.g. element 104 inFIG. 2). The term “conductive constituent” is used herein according toart accepted terminology and refers to electrically conductive sensorelements such as electrodes which are capable of measuring and adetectable signal and conducting this to a detection apparatus. Anillustrative example of this is a conductive constituent that canmeasure an increase or decrease in current in response to exposure to astimuli such as the change in the concentration of an analyte or itsbyproduct as compared to a reference electrode that does not experiencethe change in the concentration of the analyte, a coreactant (e.g.oxygen) used when the analyte interacts with a composition (e.g. theenzyme glucose oxidase) present in analyte sensing constituent 110 or areaction product of this interaction (e.g. hydrogen peroxide).Illustrative examples of such elements include electrodes which arecapable of producing variable detectable signals in the presence ofvariable concentrations of molecules such as hydrogen peroxide oroxygen. Typically one of these electrodes in the conductive constituentis a working electrode, which can be made from non-corroding metal orcarbon. A carbon working electrode may be vitreous or graphitic and canbe made from a solid or a paste. A metallic working electrode may bemade from platinum group metals, including palladium or gold, or anon-corroding metallically conducting oxide, such as ruthenium dioxide.Alternatively the electrode may comprise a silver/silver chlorideelectrode composition. The working electrode may be a wire or a thinconducting film applied to a substrate, for example, by coating orprinting. Typically, only a portion of the surface of the metallic orcarbon conductor is in electrolytic contact with the analyte-containingsolution. This portion is called the working surface of the electrode.The remaining surface of the electrode is typically isolated from thesolution by an electrically insulating cover constituent 106. Examplesof useful materials for generating this protective cover constituent 106include polymers such as polyimides, polytetrafluoroethylene,polyhexafluoropropylene and silicones such as polysiloxanes.

In addition to the working electrode, the analyte sensors of theinvention typically include a reference electrode or a combinedreference and counter electrode (also termed a quasi-reference electrodeor a counter/reference electrode). If the sensor does not have acounter/reference electrode then it may include a separate counterelectrode, which may be made from the same or different materials as theworking electrode. Typical sensors of the present invention have one ormore working electrodes and one or more counter, reference, and/orcounter/reference electrodes. One embodiment of the sensor of thepresent invention has two, three or four or more working electrodes.These working electrodes in the sensor may be integrally connected orthey may be kept separate. Optionally, the electrodes can be disposed ona single surface or side of the sensor structure. Alternatively, theelectrodes can be disposed on a multiple surfaces or sides of the sensorstructure (and can for example be connected by vias through the sensormaterial(s) to the surfaces on which the electrodes are disposed). Incertain embodiments of the invention, the reactive surfaces of theelectrodes are of different relative areas/sizes, for example a 1×reference electrode, a 2.6× working electrode and a 3.6× counterelectrode.

Typically for in vivo use, embodiments of the present invention areimplanted subcutaneously in the skin of a mammal, such as a person, fordirect contact with the body fluids of the mammal, such as blood.Alternatively the sensors can be implanted into other regions within thebody of a mammal such as in the intraperotineal space. When multipleworking electrodes are used, they may be implanted together or atdifferent positions in the body. The counter, reference, and/orcounter/reference electrodes may also be implanted either proximate tothe working electrode(s) or at other positions within the body of themammal. Embodiments of the invention include sensors comprisingelectrodes constructed from nanostructured materials. As used herein, a“nanostructured material” is an object manufactured to have at least onedimension smaller than 100 nm. Examples include, but are not limited to,single-walled nanotubes, double-walled nanotubes, multi-wallednanotubes, bundles of nanotubes, fullerenes, cocoons, nanowires,nanofibres, onions and the like.

Interference Rejection Constituent

The electrochemical sensors of the invention optionally include aninterference rejection constituent disposed between the surface of theelectrode and the environment to be assayed. In particular, certainsensor embodiments rely on the oxidation and/or reduction of hydrogenperoxide generated by enzymatic reactions on the surface of a workingelectrode at a constant potential applied. Because amperometricdetection based on direct oxidation of hydrogen peroxide requires arelatively high oxidation potential, sensors employing this detectionscheme may suffer interference from oxidizable species that are presentin biological fluids such as ascorbic acid, uric acid and acetaminophen.In this context, the term “interference rejection constituent” is usedherein according to art accepted terminology and refers to a coating ormembrane in the sensor that functions to inhibit spurious signalsgenerated by such oxidizable species which interfere with the detectionof the signal generated by the analyte to be sensed. Certaininterference rejection constituents function via size exclusion (e.g. byexcluding interfering species of a specific size). Examples ofinterference rejection constituents include one or more layers orcoatings of compounds such as hydrophilic polyurethanes, celluloseacetate (including cellulose acetate incorporating agents such aspoly(ethylene glycol), polyethersulfones, polytetra-fluoroethylenes, theperfluoronated ionomer Nafion™, polyphenylenediamine, epoxy and thelike. In particular embodiments, the interference rejection constituentsare comprised of a NAFION (a sulfonated tetrafluorethylene copolymerhaving the molecular formula C7HF13O5S. C2F4, CAS number [31175-20-9])and/or a cellulose acetate composition. Illustrative discussions of suchinterference rejection constituents are found for example in Ward etal., Biosensors and Bioelectronics 17 (2002) 181-189 and Choi et al.,Analytical Chimica Acta 461 (2002) 251-260 which are incorporated hereinby reference. Other interference rejection constituents include forexample those observed to limit the movement of compounds based upon amolecular weight range, for example cellulose acetate as disclosed forexample in U.S. Pat. No. 5,755,939, the contents of which areincorporated by reference.

An interference rejection membrane (IRM) may comprise NAFION and itseffectiveness at inhibiting interfering signals that can be generated byacetominophenol in an amperometric sensor. Typically, an IRM is disposedunder an analyte sensing layer (e.g. one comprising glucose oxidase). Incertain embodiments of the invention, the IRM is disposed between thereactive surface of an electrode and an analyte sensing layer. Relatedembodiments of the invention include methods for inhibiting one or moresignals generated by an interfering compound in various sensorembodiments of the invention (e.g. by using an interference rejectionlayer).

Analyte Sensing Constituent

The electrochemical sensors of the invention include an analyte sensingconstituent disposed on the electrodes of the sensor (see, e.g. element110 in FIG. 2). The term “analyte sensing constituent” is used hereinaccording to art accepted terminology and refers to a constituentcomprising a material that is capable of recognizing or reacting with ananalyte whose presence is to be detected by the analyte sensorapparatus. Typically this material in the analyte sensing constituentproduces a detectable signal after interacting with the analyte to besensed, typically via the electrodes of the conductive constituent. Inthis regard the analyte sensing constituent and the electrodes of theconductive constituent work in combination to produce the electricalsignal that is read by an apparatus associated with the analyte sensor.Typically, the analyte sensing constituent comprises an oxidoreductaseenzyme capable of reacting with and/or producing a molecule whose changein concentration can be measured by measuring the change in the currentat an electrode of the conductive constituent (e.g. oxygen and/orhydrogen peroxide), for example the enzyme glucose oxidase. An enzymecapable of producing a molecule such as hydrogen peroxide can bedisposed on the electrodes according to a number of processes known inthe art. The analyte sensing constituent can coat all or a portion ofthe various electrodes of the sensor. In this context, the analytesensing constituent may coat the electrodes to an equivalent degree.Alternatively the analyte sensing constituent may coat differentelectrodes to different degrees, with for example the coated surface ofthe working electrode being larger than the coated surface of thecounter and/or reference electrode.

Typical sensor embodiments of this element of the invention utilize anenzyme (e.g. glucose oxidase) that has been combined with a secondprotein (e.g. albumin) in a fixed ratio (e.g. one that is typicallyoptimized for glucose oxidase stabilizing properties) and then appliedon the surface of an electrode to form a thin enzyme constituent. In atypical embodiment, the analyte sensing constituent comprises a GOx andHSA mixture. In a typical embodiment of an analyte sensing constituenthaving GOx, the GOx reacts with glucose present in the sensingenvironment (e.g. the body of a mammal) and generates hydrogen peroxideaccording to the reaction shown in FIG. 1, wherein the hydrogen peroxideso generated is anodically detected at the working electrode in theconductive constituent.

As noted above, the enzyme and the second protein (e.g. an albumin) aretypically treated to form a crosslinked matrix (e.g. by adding across-linking agent to the protein mixture). As is known in the art,crosslinking conditions may be manipulated to modulate factors such asthe retained biological activity of the enzyme, its mechanical and/oroperational stability. Illustrative crosslinking procedures aredescribed in U.S. patent application Ser. No. 10/335,506 and PCTpublication WO 03/035891 which are incorporated herein by reference. Forexample, an amine cross-linking reagent, such as, but not limited to,glutaraldehyde, can be added to the protein mixture. The addition of across-linking reagent to the protein mixture creates a protein paste.The concentration of the cross-linking reagent to be added may varyaccording to the concentration of the protein mixture. Whileglutaraldehyde is an illustrative crosslinking reagent, othercross-linking reagents may also be used or may be used in place ofglutaraldehyde. Other suitable cross-linkers also may be used, as willbe evident to those skilled in the art.

The GOx and/or carrier protein concentration may vary for differentembodiments of the invention. For example, the GOx concentration may bewithin the range of approximately 50 mg/ml (approximately 10,000 U/ml)to approximately 700 mg/ml (approximately 150,000 U/ml). Typically theGOx concentration is about 115 mg/ml (approximately 22,000 U/ml). Insuch embodiments, the HSA concentration may vary between about 0.5%-30%(w/v), depending on the GOx concentration. Typically the HSAconcentration is about 1-10% w/v, and most typically is about 5% w/v. Inalternative embodiments of the invention, collagen or BSA or otherstructural proteins used in these contexts can be used instead of or inaddition to HSA. Although GOx is discussed as an illustrative enzyme inthe analyte sensing constituent, other proteins and/or enzymes may alsobe used or may be used in place of GOx, including, but not limited toglucose dehydrogenase or hexokinase, hexose oxidase, lactate oxidase,and the like. Other proteins and/or enzymes may also be used, as will beevident to those skilled in the art. Moreover, although HSA is employedin the example embodiment, other structural proteins, such as BSA,collagens or the like, could be used instead of or in addition to HSA.

As noted above, in some embodiments of the invention, the analytesensing constituent includes a composition (e.g. glucose oxidase)capable of producing a signal (e.g. a change in oxygen and/or hydrogenperoxide concentrations) that can be sensed by the electricallyconductive elements (e.g. electrodes which sense changes in oxygenand/or hydrogen peroxide concentrations). However, other useful analytesensing constituents can be formed from any composition that is capableof producing a detectable signal that can be sensed by the electricallyconductive elements after interacting with a target analyte whosepresence is to be detected. In some embodiments, the compositioncomprises an enzyme that modulates hydrogen peroxide concentrations uponreaction with an analyte to be sensed. Alternatively, the compositioncomprises an enzyme that modulates oxygen concentrations upon reactionwith an analyte to be sensed. In this context, a wide variety of enzymesthat either use or produce hydrogen peroxide and/or oxygen in a reactionwith a physiological analyte are known in the art and these enzymes canbe readily incorporated into the analyte sensing constituentcomposition. A variety of other enzymes known in the art can produceand/or utilize compounds whose modulation can be detected byelectrically conductive elements such as the electrodes that areincorporated into the sensor designs described herein. Such enzymesinclude for example, enzymes specifically described in Table 1, pages15-29 and/or Table 18, pages 111-112 of Protein Immobilization:Fundamentals and Applications (Bioprocess Technology, Vol 14) by RichardF. Taylor (Editor) Publisher: Marcel Dekker; Jan. 7, 1991) the entirecontents of which are incorporated herein by reference.

Other useful analyte sensing constituents can be formed to includeantibodies whose interaction with a target analyte is capable ofproducing a detectable signal that can be sensed by the electricallyconductive elements after interacting with the target analyte whosepresence is to be detected. Examples of anti-body-based apparatuses arefound in U.S. Pat. Nos. 5,427,912, 5,149,630, 6,410,251, and 4,402,819,which are incorporated herein by reference. For related disclosures, seealso U.S. Pat. Nos. 6,703,210, 5,981,203, 5,705,399 and 4,894,253, whichare incorporated herein by reference.

In addition to enzymes and antibodies, other exemplary materials for usein the analyte sensing constituents of the sensors disclosed hereininclude polymers that bind specific types of cells or cell components(e.g. polypeptides, carbohydrates and the like); single-strand DNA;antigens and the like. The detectable signal can be, for example, anoptically detectable change, such as a color change or a visibleaccumulation of the desired analyte (e.g., cells). Sensing elements canalso be formed from materials that are essentially non-reactive (i.e.,controls). The foregoing alternative sensor elements are beneficiallyincluded, for example, in sensors for use in cell-sorting assays andassays for the presence of pathogenic organisms, such as viruses (HIV,hepatitis-C, etc.), bacteria, protozoa and the like.

Also contemplated are analyte sensors that measure an analyte that ispresent in the external environment and that can in itself produce ameasurable change in current at an electrode. In sensors measuring suchanalytes, the analyte sensing constituent can be optional.

Protein Constituent

The electrochemical sensors of the invention optionally include aprotein constituent disposed between the analyte sensing constituent andthe analyte modulating constituent (see, e.g. element 116 in FIG. 2).The term “protein constituent” is used herein according to art acceptedterminology and refers to constituent containing a carrier protein orthe like that is selected for compatibility with the analyte sensingconstituent and/or the analyte modulating constituent. In typicalembodiments, the protein constituent comprises an albumin such as humanserum albumin. The HSA concentration may vary between about 0.5%-30%(w/v). Typically the HSA concentration is about 1-10% w/v, and mosttypically is about 5% w/v. In alternative embodiments of the invention,collagen or BSA or other structural proteins used in these contexts canbe used instead of or in addition to HSA. This constituent is typicallycrosslinked on the analyte sensing constituent according to art acceptedprotocols.

Adhesion Promoting Constituent

The electrochemical sensors of the invention can include one or moreadhesion promoting (AP) constituents (see, e.g. element 114 in FIG. 2).The term “adhesion promoting constituent” is used herein according toart accepted terminology and refers to a constituent that includesmaterials selected for their ability to promote adhesion betweenadjoining constituents in the sensor. Typically, the adhesion promotingconstituent is disposed between the analyte sensing constituent and theanalyte modulating constituent. Typically, the adhesion promotingconstituent is disposed between the optional protein constituent and theanalyte modulating constituent. The adhesion promoter constituent can bemade from any one of a wide variety of materials known in the art tofacilitate the bonding between such constituents and can be applied byany one of a wide variety of methods known in the art. Typically, theadhesion promoter constituent comprises a silane compound such asγ-aminopropyltrimethoxysilane.

The use of silane coupling reagents, especially those of the formulaR′Si(OR)₃ in which R′ is typically an aliphatic group with a terminalamine and R is a lower alkyl group, to promote adhesion is known in theart (see, e.g. U.S. Pat. No. 5,212,050, which is incorporated herein byreference). For example, chemically modified electrodes in which asilane such as γ-aminopropyltriethoxysilane and glutaraldehyde were usedin a step-wise process to attach and to co-crosslink bovine serumalbumin (BSA) and glucose oxidase (GOx) to the electrode surface arewell known in the art (see, e.g. Yao, T. Analytica Chim. Acta 1983, 148,27-33).

In certain embodiments of the invention, the adhesion promotingconstituent further comprises one or more compounds that can also bepresent in an adjacent constituent such as the polydimethyl siloxane(PDMS) compounds that serves to limit the diffusion of analytes such asglucose through the analyte modulating constituent. In illustrativeembodiments the formulation comprises 0.5-20% PDMS, typically 5-15%PDMS, and most typically 10% PDMS. In certain embodiments of theinvention, the adhesion promoting constituent is crosslinked within thelayered sensor system and correspondingly includes an agent selected forits ability to crosslink a moiety present in a proximal constituent suchas the analyte modulating constituent. In illustrative embodiments ofthe invention, the adhesion promoting constituent includes an agentselected for its ability to crosslink an amine or carboxyl moiety of aprotein present in a proximal constituent such a the analyte sensingconstituent and/or the protein constituent and or a siloxane moietypresent in a compound disposed in a proximal layer such as the analytemodulating layer. Optionally, a first compound in the adhesion promotinglayer is crosslinked to a second compound in the analyte sensing layer.

Analyte Modulating Constituent

The electrochemical sensors of the invention include an analytemodulating constituent disposed on the sensor (see, e.g. element 112 inFIG. 2). The term “analyte modulating constituent” is used hereinaccording to art accepted terminology and refers to a constituent thattypically forms a membrane on the sensor that operates to modulate thediffusion of one or more analytes, such as glucose, through theconstituent. In certain embodiments of the invention, the analytemodulating constituent is an analyte-limiting membrane which operates toprevent or restrict the diffusion of one or more analytes, such asglucose, through the constituents. In other embodiments of theinvention, the analyte-modulating constituent operates to facilitate thediffusion of one or more analytes, through the constituents. Optionallysuch analyte modulating constituents can be formed to prevent orrestrict the diffusion of one type of molecule through the constituent(e.g. glucose), while at the same time allowing or even facilitating thediffusion of other types of molecules through the constituent (e.g. O₂).

With respect to glucose sensors, in known enzyme electrodes, glucose andoxygen from blood, as well as some interferants, such as ascorbic acidand uric acid, diffuse through a primary membrane of the sensor. As theglucose, oxygen and interferants reach the analyte sensing constituent,an enzyme, such as glucose oxidase, catalyzes the conversion of glucoseto hydrogen peroxide and gluconolactone. The hydrogen peroxide maydiffuse back through the analyte modulating constituent, or it maydiffuse to an electrode where it can be reacted to form oxygen and aproton to produce a current that is proportional to the glucoseconcentration. The sensor membrane assembly serves several functions,including selectively allowing the passage of glucose therethrough. Inthis context, an illustrative analyte modulating constituent is asemi-permeable membrane which permits passage of water, oxygen and atleast one selective analyte and which has the ability to absorb water,the membrane having a water soluble, hydrophilic polymer.

A variety of illustrative analyte modulating compositions are known inthe art and are described for example in U.S. Pat. Nos. 6,319,540,5,882,494, 5,786,439 5,777,060, 5,771,868 and 5,391,250, the disclosuresof each being incorporated herein by reference. The hydrogels describedtherein are particularly useful with a variety of implantable devicesfor which it is advantageous to provide a surrounding water constituent.In some embodiments of the invention, the analyte modulating compositionincludes PDMS. In certain embodiments of the invention, the analytemodulating constituent includes an agent selected for its ability tocrosslink a siloxane moiety present in a proximal constituent. Inclosely related embodiments of the invention, the adhesion promotingconstituent includes an agent selected for its ability to crosslink anamine or carboxyl moiety of a protein present in a proximal constituent.

In some embodiments of the invention, a hydrophilic analyte modulatinglayer is coated over at least 50, 75% or 100% of the reactive surface ofan electrode (e.g. an electrically conductive wire).

Cover Constituent

The electrochemical sensors of the invention include one or more coverconstituents which are typically electrically insulating protectiveconstituents (see, e.g. element 106 in FIG. 2). Typically, such coverconstituents can be in the form of a coating, sheath or tube and aredisposed on at least a portion of the analyte modulating constituent.Acceptable polymer coatings for use as the insulating protective coverconstituent can include, but are not limited to, non-toxic biocompatiblepolymers such as silicone compounds, polyimides, biocompatible soldermasks, epoxy acrylate copolymers, or the like. Further, these coatingscan be photo-imageable to facilitate photolithographic forming ofapertures through to the conductive constituent. A typical coverconstituent comprises spun on silicone. As is known in the art, thisconstituent can be a commercially available RTV (room temperaturevulcanized) silicone composition. A typical chemistry in this context ispolydimethyl siloxane (acetoxy based).

C. Typical Analyte Sensor System Embodiments of the Invention

Embodiments of the sensor elements and sensors can be operativelycoupled to a variety of other systems elements typically used withanalyte sensors (e.g. structural elements such as piercing members,insertion sets and the like as well as electronic components such asprocessors, monitors, medication infusion pumps and the like), forexample to adapt them for use in various contexts (e.g. implantationwithin a mammal). One embodiment of the invention includes a method ofmonitoring a physiological characteristic of a user using an embodimentof the invention that includes an input element capable of receiving asignal from a sensor that is based on a sensed physiologicalcharacteristic value of the user, and a processor for analyzing thereceived signal. In typical embodiments of the invention, the processordetermines a dynamic behavior of the physiological characteristic valueand provides an observable indicator based upon the dynamic behavior ofthe physiological characteristic value so determined. In someembodiments, the physiological characteristic value is a measure of theconcentration of blood glucose in the user. In other embodiments, theprocess of analyzing the received signal and determining a dynamicbehavior includes repeatedly measuring the physiological characteristicvalue to obtain a series of physiological characteristic values in orderto, for example, incorporate comparative redundancies into a sensorapparatus in a manner designed to provide confirmatory information onsensor function, analyte concentration measurements, the presence ofinterferences and the like.

Embodiments of the invention include devices which display data frommeasurements of a sensed physiological characteristic (e.g. bloodglucose concentrations) in a manner and format tailored to allow a userof the device to easily monitor and, if necessary, modulate thephysiological status of that characteristic (e.g. modulation of bloodglucose concentrations via insulin administration). An illustrativeembodiment of the invention is a device comprising a sensor inputcapable of receiving a signal from a sensor, the signal being based on asensed physiological characteristic value of a user; a memory forstoring a plurality of measurements of the sensed physiologicalcharacteristic value of the user from the received signal from thesensor; and a display for presenting a text and/or graphicalrepresentation of the plurality of measurements of the sensedphysiological characteristic value (e.g. text, a line graph or the like,a bar graph or the like, a grid pattern or the like or a combinationthereof). Typically, the graphical representation displays real timemeasurements of the sensed physiological characteristic value. Suchdevices can be used in a variety of contexts, for example in combinationwith other medical apparatuses. In some embodiments of the invention,the device is used in combination with at least one other medical device(e.g. a glucose sensor).

An illustrative system embodiment consists of a glucose sensor, atransmitter and pump receiver and a glucose meter. In this system, radiosignals from the transmitter can be sent to the pump receiver every 5minutes to provide providing real-time sensor glucose (SG) values.Values/graphs are displayed on a monitor of the pump receiver so that auser can self monitor blood glucose and deliver insulin using their owninsulin pump. Typically an embodiment of device disclosed hereincommunicates with a second medical device via a wired or wirelessconnection. Wireless communication can include for example the receptionof emitted radiation signals as occurs with the transmission of signalsvia RF telemetry, infrared transmissions, optical transmission, sonicand ultrasonic transmissions and the like. Optionally, the device is anintegral part of a medication infusion pump (e.g. an insulin pump).Typically in such devices, the physiological characteristic valuesinclude a plurality of measurements of blood glucose.

FIG. 3 provides a perspective view of one generalized embodiment ofsubcutaneous sensor insertion system and a block diagram of a sensorelectronics device according to one illustrative embodiment of theinvention. Additional elements typically used with such sensor systemembodiments are disclosed for example in U.S. Patent Application No.20070163894, the contents of which are incorporated by reference. FIG. 3provides a perspective view of a telemetered characteristic monitorsystem 1, including a subcutaneous sensor set 10 provided forsubcutaneous placement of an active portion of a flexible sensor 12, orthe like, at a selected site in the body of a user. The subcutaneous orpercutaneous portion of the sensor set 10 includes a hollow, slottedinsertion needle 14 having a sharpened tip 44, and a cannula 16. Insidethe cannula 16 is a sensing portion 18 of the sensor 12 to expose one ormore sensor electrodes 20 to the user's bodily fluids through a window22 formed in the cannula 16. The sensing portion 18 is joined to aconnection portion 24 that terminates in conductive contact pads, or thelike, which are also exposed through one of the insulative layers. Theconnection portion 24 and the contact pads are generally adapted for adirect wired electrical connection to a suitable monitor 200 coupled toa display 214 for monitoring a user's condition in response to signalsderived from the sensor electrodes 20. The connection portion 24 may beconveniently connected electrically to the monitor 200 or acharacteristic monitor transmitter 100 by a connector block 28 (or thelike) as shown and described in U.S. Pat. No. 5,482,473, entitled FLEXCIRCUIT CONNECTOR, which is incorporated by reference. In typicalembodiments of the invention, a contact pad and an electrode are atleast, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25 millimeters apart.

As shown in FIG. 3, in accordance with embodiments of the presentinvention, subcutaneous sensor set 10 may be configured or formed towork with either a wired or a wireless characteristic monitor system.The proximal part of the sensor 12 is mounted in a mounting base 30adapted for placement onto the skin of a user. The mounting base 30 canbe a pad having an underside surface coated with a suitable pressuresensitive adhesive layer 32, with a peel-off paper strip 34 normallyprovided to cover and protect the adhesive layer 32, until the sensorset 10 is ready for use. The mounting base 30 includes upper and lowerlayers 36 and 38, with the connection portion 24 of the flexible sensor12 being sandwiched between the layers 36 and 38. The connection portion24 has a forward section joined to the active sensing portion 18 of thesensor 12, which is folded angularly to extend downwardly through a bore40 formed in the lower base layer 38. Optionally, the adhesive layer 32(or another portion of the apparatus in contact with in vivo tissue)includes an anti-inflammatory agent to reduce an inflammatory responseand/or anti-bacterial agent to reduce the chance of infection. Theinsertion needle 14 is adapted for slide-fit reception through a needleport 42 formed in the upper base layer 36 and through the lower bore 40in the lower base layer 38. After insertion, the insertion needle 14 iswithdrawn to leave the cannula 16 with the sensing portion 18 and thesensor electrodes 20 in place at the selected insertion site. In thisembodiment, the telemetered characteristic monitor transmitter 100 iscoupled to a sensor set 10 by a cable 102 through a connector 104 thatis electrically coupled to the connector block 28 of the connectorportion 24 of the sensor set 10.

In the embodiment shown in FIG. 3, the telemetered characteristicmonitor 100 includes a housing 106 that supports a printed circuit board108, batteries 110, antenna 112, and the cable 102 with the connector104. In some embodiments, the housing 106 is formed from an upper case114 and a lower case 116 that are sealed with an ultrasonic weld to forma waterproof (or resistant) seal to permit cleaning by immersion (orswabbing) with water, cleaners, alcohol or the like. In someembodiments, the upper and lower case 114 and 116 are formed from amedical grade plastic. However, in alternative embodiments, the uppercase 114 and lower case 116 may be connected together by other methods,such as snap fits, sealing rings, RTV (silicone sealant) and bondedtogether, or the like, or formed from other materials, such as metal,composites, ceramics, or the like. In other embodiments, the separatecase can be eliminated and the assembly is simply potted in epoxy orother moldable materials that is compatible with the electronics andreasonably moisture resistant. As shown, the lower case 116 may have anunderside surface coated with a suitable pressure sensitive adhesivelayer 118, with a peel-off paper strip 120 normally provided to coverand protect the adhesive layer 118, until the sensor set telemeteredcharacteristic monitor transmitter 100 is ready for use.

In the illustrative embodiment shown in FIG. 3, the subcutaneous sensorset 10 facilitates accurate placement of a flexible thin filmelectrochemical sensor 12 of the type used for monitoring specific bloodparameters representative of a user's condition. The sensor 12 monitorsglucose levels in the body, and may be used in conjunction withautomated or semi-automated medication infusion pumps of the external orimplantable type as described in U.S. Pat. No. 4,562,751; 4,678,408;4,685,903 or 4,573,994, to control delivery of insulin to a diabeticpatient.

In the illustrative embodiment shown in FIG. 3, the sensor electrodes 10may be used in a variety of sensing applications and may be configuredin a variety of ways. For example, the sensor electrodes 10 may be usedin physiological parameter sensing applications in which some type ofbiomolecule is used as a catalytic agent. For example, the sensorelectrodes 10 may be used in a glucose and oxygen sensor having aglucose oxidase enzyme catalyzing a reaction with the sensor electrodes20. The sensor electrodes 10, along with a biomolecule or some othercatalytic agent, may be placed in a human body in a vascular ornon-vascular environment. For example, the sensor electrodes 20 andbiomolecule may be placed in a vein and be subjected to a blood stream,or may be placed in a subcutaneous or peritoneal region of the humanbody.

In the embodiment of the invention shown in FIG. 3, the monitor ofsensor signals 200 may also be referred to as a sensor electronicsdevice 200. The monitor 200 may include a power source, a sensorinterface, processing electronics (i.e. a processor), and dataformatting electronics. The monitor 200 may be coupled to the sensor set10 by a cable 102 through a connector that is electrically coupled tothe connector block 28 of the connection portion 24. In an alternativeembodiment, the cable may be omitted. In this embodiment of theinvention, the monitor 200 may include an appropriate connector fordirect connection to the connection portion 104 of the sensor set 10.The sensor set 10 may be modified to have the connector portion 104positioned at a different location, e.g., on top of the sensor set tofacilitate placement of the monitor 200 over the sensor set.

D. Embodiments of the Invention and Associated Characteristics

Embodiments of the invention disclosed herein focus on implantableanalyte sensors and sensor systems that are designed to include elementsand/or configurations of elements that facilitate sensor initializationand/or start-up times in vivo, for example, the time that it takes for asensor to settle into its environment (e.g. become appropriatelyhydrated), and/or begin to sense analyte concentrations and/or starttransmitting meaningful information to a user. As discussed furtherherein, it is known in the art that the amount time required for sensorinitialization and/or start-up prior to its use can be relatively long(e.g. in amperometric glucose sensors, the sensor start-upinitialization times can range from 2 to 10 hours), a factor which canhinder the use of such sensors in the administration of medical care.For example, in hospital settings, a relatively long sensorinitialization and/or start-up period can delay the receipt of importantinformation relating to patient health (e.g. hyperglycemia orhypoglycemia in a diabetic patient), thereby delaying treatmentspredicated on the receipt of such information (e.g. the administrationof insulin).

In addition, a relatively long sensor initialization and/or start-upperiod in hospital settings can require repeated monitoring by hospitalstaff, a factor which contributes to the costs of patient care.Moreover, these long initialization times can also be a problem if asensor needs to be removed from sensor electronics and then connectedagain, for example for an MRI procedure where the sensor electronics arenot compatible. In this context, electronic processing and/ortelemetering is typically employed with the amperometric sensors, whichare, for example, useful for buffering the electrical signals producedby the sensors, processing the sensor signals for transmission, andcommunicating the buffered, processing signals via a link to amonitoring unit etc.

Sensors having reduced initialization and/or start-up times in vivo inhospital settings and sensors and sensor systems that are designed toinclude elements and/or configurations of elements that diminish longsensor initialization and/or start-up times are highly desirable. Withglucose sensors for example, even a 15-30 minute reduction of sensorinitialization and/or start-up time is highly desirable because, forexample, such shorter initialization times can: (1) reduce the need forpatient monitoring by hospital personnel, a factor which contributes tothe cost-effectiveness of such medical devices; and (2) reduce delays inthe receipt of important information relating to patient health. It isfurther desirable to reduce the initialization time even further forsensors that are already in the body of a patient but have beendisconnected from sensor electronics for a short amount of time, such asless than 2 hours.

In individuals using analyte sensors in non-hospital settings (e.g.diabetics using glucose sensors to manage their disease), relativelylong sensor initialization and/or start-up periods are alsoproblematical due to both the inconvenience to the user as well as thedelayed receipt of information relating to user health. Because manydiabetics do not have medical training, they may forgo optimalmonitoring and modulation of blood glucose levels due to complexitiesassociated with such management, for example, a two hour start-up periodwhich can be an inconvenience in view of a patient's active dailyroutine. For these reasons, sensors and sensor systems that are designedto include elements and/or configurations of elements can reduce sensorinitialization and/or start-up times in are highly desirable insituations where such sensors are operated by a diabetic patient withoutmedical training because they facilitate the patient's convenientmanagement of their disease, behavior which is shown to decrease thewell known morbidity and mortality issues observed in individualssuffering from chronic diabetes.

While the analyte sensor and sensor systems disclosed herein aretypically designed to be implantable within the body of a mammal, theinventions disclosed herein are not limited to any particularenvironment and can instead be used in a wide variety of contexts, forexample for the analysis of most in vivo and in vitro liquid samplesincluding biological fluids such as interstitial fluids, whole-blood,lymph, plasma, serum, saliva, urine, stool, perspiration, mucus, tears,cerebrospinal fluid, nasal secretion, cervical or vaginal secretion,semen, pleural fluid, amniotic fluid, peritoneal fluid, middle earfluid, joint fluid, gastric aspirate or the like. In addition, solid ordesiccated samples may be dissolved in an appropriate solvent to providea liquid mixture suitable for analysis.

In certain embodiments of the invention, distributed electrodeconfigurations are used in methods designed to overcome problems withsensors and sensor systems that occur due to lack of hydration (e.g.slow start-up initialization times), fluid stagnation, a patient'simmune response, or the like. For example, systems with such distributedelectrode configurations are shown in U.S. Pat. No. 6,770,729, which isincorporated herein by reference.

In some sensor embodiments, the distributed electrodes areorganized/disposed within a flex-circuit assembly (i.e. a circuitryassembly that utilizes flexible rather than rigid materials). Suchflex-circuit assembly embodiments provide an interconnected assembly ofelements (e.g. electrodes, electrical conduits, contact pads and thelike) configured to facilitate wearer comfort (for example by reducingpad stiffness and wearer discomfort) as well as parameter measurementperformance and are disclosed in more detail in U.S. patent applicationSer. No. 12/184,046 (filed Jul. 31, 2008), which is hereby incorporatedby reference.

Typically, the electrodes in a sensor are of a rectangular shape, i.e.have a longer side and a shorter side (including those of a rectangularshape, yet having rounded edges). In some embodiments of the invention,the electrode configuration is such that a longer side of at least oneof the electrodes in a distributed electrode pattern is parallel to anlonger side of at least one of the other electrodes in the distributedelectrode pattern (and optionally all of the electrodes in thedistributed electrode pattern). Example sensors are shown in U.S. patentapplication Ser. No. 12/184,046 (filed Jul. 31, 2008), incorporatedherein by reference.

In some embodiments of the invention, an aperture is positioned on thecover layer so that a fluid comprising the analyte contacts thereference electrode, the working electrode and the counter electrode ina sequential manner so as to facilitate sensor hydration and/or sensorstart-up or initialization. In some embodiments of the invention, theaperture is fully open, i.e. opens the electrodes to the externalenvironment by having aperture edges that line up with or are below theelectrodes in the sensor. An optimized profile is shown in U.S. patentapplication Ser. No. 12/184,046 (filed Jul. 31, 2008), incorporatedherein by reference.

In certain embodiments of the invention, sensor systems that comprisewire electrodes are used in methods designed to overcome problems thatcan occur with implantable sensors and sensor systems due to lack ofhydration (e.g. slow start-up initialization times) and/or fluidstagnation by enhancing the flexing and movement of the implantedcomponents in a manner that enhances fluid flow and inhibit a gas bubbleor a stagnating pool of fluid from remaining on top of or close to anelectrode in a manner that compromises sensor function. In addition,embodiments of the invention that comprise a wire electrodes can becombined with certain complementary elements disclosed herein so as tofurther overcome problems that result from a lack of hydration, fluidstagnation, a patient's immune response, or the like (e.g. distributedelectrode configurations, flex sensor assemblies, multiple electrodesensors, voltage pulsing methods etc.).

As discussed herein, the sensor may be directly connected to sensorelectronics, which may be part of or separately connected (wirelessly orvia or other direct connection) to a monitoring device that monitors thesignals received from the sensor. Depending on the construction of thesensor device and/or monitor (whether separate or together with thesensor device), one or both of the sensor electronics and monitor maymake calculations based on the signals sensed at the sensor to convertthe signals to actual analyte measurements and to determine variouscharacteristics of the data received. As discussed herein, a number ofdifferent characteristics may be used to help get a more accuratepicture of the actual level of analyte in the patient. Thesecharacteristics can include current values at different time intervals,such as during relaxation of the curve, change in currents, change intotal charge, and/or calculated relaxation parameters.

Embodiments of the invention include sensors and sensor systems havingconfigurations of elements and/or architectures that optimize aspects ofsensor function. For example, certain embodiments of the invention areconstructed to include multiple and/or redundant elements such asmultiple sets of sensors and/or sensor system elements such as multiplepiercing members (e.g. needles) and/or a cannulas organized on aninsertion apparatus for use at a patient's in vivo insertion site. Forexample, sensor sets may include dual piercing members as disclosed inU.S. patent application Ser. No. 13/008,723, filed Jan. 18, 2011, whichis herein incorporated by reference.

In some embodiments of the invention, the first and secondelectrochemical sensors are operatively coupled to a sensor inputcapable of receiving signals from the first and second electrochemicalsensors; and a processor coupled to the sensor input, wherein theprocessor is capable of characterizing one or more signals received fromthe first and second electrochemical sensors. Optionally, a pulsedvoltage is used to obtain a signal from an electrode. In certainembodiments of the invention, the processor is capable of comparing afirst signal received from a working electrode in response to a firstworking potential with a second signal received from a working electrodein response to a second working potential.

While embodiments of the invention can comprise one or two piercingmembers, optionally such sensor apparatuses can include 3 or 4 or 5 ormore piercing members that are coupled to and extend from a base elementand are operatively coupled to 3 or 4 or 5 or more electrochemicalsensors (e.g. microneedle arrays, embodiments of which are disclosed forexample in U.S. Pat. Nos. 7,291,497 and 7,027,478, and U.S. patentApplication No. 20080015494, the contents of which are incorporated byreference). In addition, while embodiments of the invention typicallyinclude a base element that positions and supports the implantedsensors, in alternative embodiments of the invention, the plurality ofsensors are not coupled to a base element.

As noted above, certain embodiments of the invention can use voltageswitching as part of the sensing process. Embodiments of the inventioncan use voltage switching not only in the detection of interferingspecies and/or specific analyte concentrations but also to facilitatethe hydration and/or initialization of various sensor embodiments of theinvention. In particular, the time for initialization (“run-in”) differsfor different sensors and can take hours. Embodiments of the inventioninclude a sensor initialization scheme involving high frequencyinitialization (switching of voltage potentials). In one illustrativeembodiment, a triple initialization profile is used where the voltage ofthe sensor is switched between a first potential such as 0, 280, 535,635 or 1.070 millivolts and a second potential such as 0, 280, 535, 635or 1.070 millivolts over a period of 5, 10, 20, 30 or 45 seconds or 1,5, 10 or 15 minutes. Certain voltage switching embodiments of theinvention further use voltage pulsing in the detection of analytesignals. The number of pulses used in such embodiments of the inventionis typically at least 2 and can be 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 ormore. Pulses can be for a predetermined period of time, for example 1,3, 5, 7, 10, 15, 30, 45, 60, 90 or 120 seconds. One illustrative exampleof this comprises 6 pulses, each a few seconds long. By using suchembodiments of the invention, the sensor run-in is greatly accelerated,a factor which optimizes a user's introduction and activation of thesensor. Certain of these methods can be adapted for use with similarmethods known in the art (see, e.g. U.S. Pat. Nos. 5,320,725; 6,251,260and U.S. Patent Application No. 2005/0161346, the content of which areincorporated by reference).

In some embodiments of the invention, a pulsed (e.g. produced ortransmitted or modulated in short bursts or pulses) voltage is used toobtain a signal from one or more electrodes of the sensor. In relatedembodiments of the invention, the use of a pulsed current or the like isused. Such pulsing for example can be used to reduce/compensate forbackground current readings. Pulsing allows sensors to detect lowerconcentrations of glucose more efficiently, that there is a linearresponse to glucose switching, and that pulsing can be used to bothdecrease the background current and reduce the effect of interferants.To eliminate the possibility of the sensors in these studies having avariety of different voltage pulsed and/or voltage switched sensorembodiments are contemplated. In this context, sensor systems caninclude a processor in or separate from sensor electronics, where theprocessor includes software algorithms that control factors such asvoltage output and/or working potential and/or pulsing and or switchingand/or the time periods of such factors. Sensor systems can also includevarious hardware features designed to facilitate voltage pulsing, forexample discharge circuit elements. In particular, in certainembodiments of the invention, high frequency switching can require adischarge circuit element so that layers discharge held charge (whereinthe sensor layers analogous to a capacitor). One illustrative embodimentis sensor having two specific potential dedicated electrodes (e.g. at280 my and 535 my) and is designed to obtain readings of both electrodesas sensor switches between them. In this context, it is known in art totake sensor reading at a wide range of potentials (see, e.g. U.S. Pat.Nos. 5,320,725, 6,251,260, 7,081,195 and Patent Application No.2005/0161346). In one illustrative embodiment of the invention, aprocessor is used to observing signals obtained from one of two workingelectrodes in a sensor via a pulsed voltage and comparing it to thesignal obtained from the second working electrode, wherein this secondworking electrode is not exposed to a pulsed voltage.

In certain embodiments of the invention, sensor systems that utilizevoltage pulsing and/or switching as disclosed herein are used in methodsdesigned to overcome problems that can occur with implantable sensorsand sensor systems due to lack of hydration (e.g. slow start-upinitialization times) and/or fluid stagnation by enhancing the abilityof a fluid to flow around the implanted components in a manner thatinhibits the likelihood of a gas bubble or a stagnating pool of fluidfrom forming and/or remaining on top of or close to an electrode in amanner that compromises sensor function. In addition, embodiments of theinvention that utilize voltage pulsing and/or switching can be combinedwith certain complementary elements disclosed herein so as to furtherovercome problems that result from a lack of hydration, fluidstagnation, a patient's immune response, or the like (e.g. distributedelectrode configurations, multiple electrode sensors, multiple sensorapparatuses having multiple implantation sites, etc.).

In embodiments of the invention, varied voltage is used, for exampleapplying repeated cycles of step electrode potentials. The variedvoltage results in a continuous mode of glucose sensing providing muchmore information during chronological glucose monitoring. Using avarying voltage scheme such as a stepped voltage scheme has manyadvantages. For example, its inherent self-correlation is quite largecompared to a constant potential approach.

When step electrode potentials are used, for example each waveform cycleof signal relaxation response that is obtained contains a number ofcharacteristic electrode current readings (Isigs). These readings changeand relaxation times will directly correlate with glucoseconcentrations. Continuous repetition of such cycles results in a robustcontinuous glucose monitoring system. The characteristic signalresponses, by correlating to glucose, also correlate with each otherunder normal conditions throughout any glucose changes. Thus, thismethod provides higher system reliability as compared to a fixedpotential sensing mode, which only returns one characteristic electrodecurrent reading during sensing. Changes in system self-correlation basedon multiple electrode potentials can also be useful in identifying thepresence of substances that may interfere with glucose response andtracking such as interferants. In embodiments of the invention, multipleelectrode potentials are used, for example, stepped electrodepotentials.

As discussed herein, methods such as voltage switching may be used toinitialize the sensor prior to the time at which sensing data will beused to determine analyte readings. As such, there may be aninitialization period prior to the sensor duration time period. Inaddition or alternatively, also as discussed herein, there may be ahydration period prior to the sensor duration time period.

In some cases, the sensor needs to be disconnected from the sensorelectronics for a period and then reconnected. For example, inhospitals, a sensor may need to be disconnected during certainoperations or procedures. One such procedure is magnetic resonanceimaging (MRI), which is not compatible with processors or sensorelectronics. In such cases, the sensor is disconnected for a certainamount of time and then reconnected to the sensor electronics.Unfortunately, the prior art processes of initialization can take up toa couple of hours. During normal operation in certain embodiments, thesensor operates in an amperometric detection mode at a steady operationpotential (e.g., 0.535 V) to provide a steady-state faradaic currentresponse that corresponds to the glucose concentrations. In an eventwhen the sensor needs to be disconnected and reconnected for a briefperiod of time, the sensor undergoes transient current response which isnon-faradaic and does not truly correspond to actual glucose values. Thetypical response curve is shown in FIG. 9A, which shows the currentresponse (iSig) in nA over a period of time. For an implanted sensor,this transient response can cause delayed start-up after reconnecting.The transient current response could last from several minutes to acouple of hours before the sensor is stabilized. The length of time forthis transient current also depends on how long the sensor wasdisconnected.

To minimize the delayed start-up upon reconnection of the sensor,embodiments of the present invention use a soft initialization method.In this soft initialization method, voltage steps or other variationsare applied in sequences that will minimize the time to reach steadystate current. The typical response curve after soft initialization isshown in FIG. 9B. As can be seen, and as discussed further herein, thetime for the response to normalize is much less than without the softinitialization method.

In embodiments of the invention, a method is provided for detectingdisconnection and reconnection of the sensor electronics processor tothe sensor. Current output to the sensor electronics processor ismeasured. When the current output is below a disconnection threshold, anevent of disconnection is determined. The threshold may be, for example,0.6 nA. After a disconnection event, the current output level to thesensor electronics continues to be measured. When it rises above areconnection threshold, an event of reconnection is determined. Thethreshold may be the same as the disconnection level or it may besmaller, for example, 0.4 nA. In embodiments of the invention, thethreshold disconnection level may be any level up to about 1.0 μA.

When the events of disconnection and reconnection are measured, theamount of time of disconnection may also be measured. If thisdisconnection time falls within certain ranges, the system of thepresent invention may select a type of initialization. For example,there may be two or three disconnection time ranges. It is possible thatthere could me more disconnection time ranges if fourth, fifth, or moretypes of initializations are desired.

Typical embodiments of the invention comprise a method of initializing asensor (typically an analyte sensor, such as a glucose sensor) bydetermining a disconnection time, wherein the disconnection time is theamount of time a sensor has been disconnected from sensor electronics.In such embodiments, an initialization protocol is then selected basedon the disconnection time. In embodiments of the invention, thedisconnection time can be characterized by determining if it fallswithin a selected time range. In some embodiments of the invention, timerange can comprise times above and/or below a specific time point that asensor has been disconnected from sensor electronics, for example,sensors having been disconnected from sensor electronics for at least 5,10, 15, 30, 60, 90 or 120 minutes (e.g. a time range of 5 minutes toinfinity etc.), or sensors having been disconnected from sensorelectronics for less than 5, 10, 15, 30, 60, 90 or 120 minutes (e.g. atime range of 0-5 minutes etc.). In certain embodiments of theinvention, time ranges can comprise windows of time that a sensor hasbeen disconnected from sensor electronics, for example, sensors havingbeen disconnected from sensor electronics from between 1-5 minutes, 1-10minutes, 5-10 minutes, 5-15 minutes, 10-30 minutes, 10-60 minutes, 10-90minutes or 10-120 minutes etc.

In typical embodiments of the invention, the initialization protocol isselected from the group consisting of: a first initialization schemecomprising a first series of voltage pulses and a second initializationscheme comprising a second series of voltage pulses, wherein the firstinitialization scheme is selected if the disconnection time falls withina first time range and the second initialization scheme is selected ifthe disconnection time falls within a second time range; and applyingthe selected initialization protocol to the sensor. The initializationprotocol may further be selected from the group consisting of a thirdinitialization scheme comprising the application of no voltage to thesensor, wherein the third initialization scheme is selected if thedisconnection time is less than the first time range and the second timerange. Alternatively, the third initialization scheme may be selected ifthe disconnection time falls within a third time range. There may beadditional initialization schemes and time ranges as desired. In anexample embodiment, the first initialization scheme is a hardinitialization scheme, the second initialization scheme is a softinitialization scheme, and the third initialization scheme is a novoltage initialization scheme, all three of which are discussed herein.

As noted above, in embodiments of the present invention there are threedisconnection time ranges, and three respective types of initialization.These three types of initialization include a hard initializationscheme, for those sensors disconnected for more than a certain amount oftime, such as 2 hours, a soft initialization scheme, for those sensorsdisconnected less than that first amount of time but more than a smalleramount of time, such as 10 minutes, and a no voltage initializationscheme for those sensors disconnected less than the smaller amount oftime. This allows for sensors having been disconnected long enough toneed a complete, hard initialization, sensors that can be initializedusing an intermediate, soft initialization, and sensors that reallydon't need an initialization because they've been disconnected for sucha short period of time. In further embodiments, there might be only thehard and no voltage initiation schemes, or only hard and soft, or onlysoft and no voltage initialization.

In embodiments of the present invention, there are three disconnectiontime ranges, and three respective types of initialization. As shown inFIG. 4, when a sensor is connected to sensor electronics at step 401 thesystem determines whether the sensor is a new sensor or a reconnectedsensor at step 405. If the sensor is a new sensor, the system goes toinitialization scheme 1 at step 430 and FIG. 5. If the sensor is not anew sensor, the system determines whether the sensor disconnection timeis within a certain range, for example less than 10 minutes, at step410. If the sensor disconnection time is within that range, theninitialization scheme 3 is selected at step 415 and FIG. 7. If thesensor disconnection time is not within that range, then it isdetermined whether the sensor disconnection time is within a seconddisconnection range, for example 10-120 minutes, at step 420. If thesensor disconnection time is within that range, then initializationscheme 2 is selected at step 425 and FIG. 6. If the sensor disconnectiontime is not within that range, then it will fall within the remainingrange of time, for example greater than 120 minutes, and initializationscheme 1 is initialized at step 430 and FIG. 5.

Although a particular selection process is shown in FIG. 4, it ispossible that the particular process could be different. For example,the disconnection time could be compared to a lookup table or it couldbe steps in a different order, however a programmer sees suitable toprepare the process such that one of three initialization schemes isselected based on the disconnection time ranges.

A sample hard initialization scheme, initialization scheme 1 is shown inFIG. 5. Under embodiments of the invention, a hard initialization isused for new sensors or when the sensor has been disconnected from thesensor electronics for greater than a certain amount of time. Initially,hydration of the sensor may be performed at step 501. Hydrationtechniques may be conventional techniques or other techniques describedherein. After hydration, if performed, a hard voltage initialization isperformed at step 505. The hard voltage initialization may be performedusing voltage switching, pulsing, or stepping, as described herein. Anexample hard voltage initialization is shown in FIG. 10. After up to 5minutes of hydration, a voltage switching scheme is employed until theminute mark. Thus, in the embodiment shown in FIG. 10, if 5 minutes ofhydration is performed, 15 minutes of hard initialization voltageswitching is performed, switching between 1.07V for 2 minutes and −0.55Vfor 1 minute. The particular embodiment in the figure is illustrativeand different voltages and time periods could also be used. For example,additional voltages could be positive or negative 0, 280, 535, 585, 635and 1.070 mV.

After the hard voltage initialization at 505, stabilization is performedat step 510. The stabilization may be performed by applying theoperating voltage, for example 535 mV, for a certain amount of time, forexample 16 minutes. At step 515, the system determines whether thesensor is stable. Metrics of glucose sensor signal stability are knownin the art and described, for example, in WO/2011/163294. In someembodiments, sensor stability is determined, for example, by determiningwhether the sensor exhibits a fixed current profile in the presence ofunchanging glucose concentrations. In some embodiments, sensor stabilityis determined, for example, by determining whether the sensor hasachieved 90% of a maximum unchanged signal in the presence of unchangingglucose concentrations. In some embodiments, sensor stability isdetermined, for example, by determining whether the sensor exhibitslimiting current in the presence of unchanging glucose concentrations.

If the sensor is not stable (e.g. does not exhibits a fixed currentprofile in the presence of unchanging glucose concentrations), thesystem performs additional stabilization until a maximum stabilizationtime or a maximum additional stabilization time is reached (e.g. apreselected time for stabilizing the sensor such as 15, 30, 45, 60, 90,120, 180 or more minutes). In the embodiment shown in FIG. 5, the systemdetermines whether there has been stabilization for less than a maximumstabilization time, such as 30 minutes, at step 515. If not, thenadditional stabilization is performed at step 510. The additionalstabilization can be for the same amount of time as the originalstabilization time or a different amount of time. For example, it mightbe useful to have a smaller secondary stabilization time because thesensor has already been stabilizing for a period of time and may justneed a short amount of time to stabilize. Also, the system could be setup to have a series of small stabilization times, such as 1 or 5 minutesfollowed by stabilization checks. In other embodiments, thestabilization check could be going on during the stabilization periodsuch that there is not a loop type stabilization process but instead acontinuous stabilization with stabilization checks until stabilizationis reached. If the sensor does not become stabilized after a maximumamount of time, the sensor is not useable and should be removed andreplaced. This is shown in FIG. 5 as step 525. As shown in FIG. 5, ifthe sensor is stable, it is calibrated at step 530. Calibration may beperformed using a blood glucose meter as described in U.S. patentapplication Ser. No. 09/334,996, filed Jun. 17, 1999, entitled“Characteristic Monitor With A Characteristic Meter and Method of Usingthe Same,” which is incorporated by reference herein, and U.S. patentapplication Ser. No. 11/931,866, filed Oct. 31, 2007, entitled “ModifiedSensor Calibration Algorithm,” which is also incorporated by referenceherein or by other calibration methods. Traditional calibration methodsuse a real time glucose value taken by blood glucose meter using thetraditional finger-prick method (and analysis of the blood takentherefrom) and using that real value to calibrate the values beingobtained by the sensor inside the body and related sensor electronics.These methods or other calibration methods may be used with theembodiments discussed herein.

After calibration, sensing may begin at step 535. It is furtherpossible, although it may not be as efficient, that there may be astabilization period with no check whether or not the sensor is stableat the end of the stabilization period.

A sample soft voltage initialization scheme, initialization scheme 2, isshown in FIG. 6. The soft initialization is used for sensors that havebeen previously connected to sensor electronics and are beingreconnected. In certain embodiments, such as shown in FIG. 4, the softinitialization is not used for sensors that have been disconnected formore than a predetermined amount of time, such as 2 hours. However, itis possible that a soft initialization may be useful for anydisconnected sensor that is being reconnected, without a maximumdisconnection period of time. With or without the maximum disconnectiontime, there may also be a minimum disconnection time for using the softinitialization, such as 10 minutes.

In the embodiment shown in FIG. 6, when initialization scheme 2 isselected, the soft voltage initialization is applied at step 601. It isgenerally not necessary to include a hydration step prior to the softvoltage initialization, because the sensor should remain inside a user'sbody. However, it is possible to do so if desired, for example if forsome reason the user removes a replaceable sensor from the body and thenreplaces it prior to reconnection. In embodiments of the invention, thesoft initialization procedure involves a series of potential steps tothe sensor using a processor. In certain embodiments, the number ofpotential steps could range from 2 to 20 steps. It is also possible tohave more of a voltage switching or pulsing type initialization. In someembodiments of the invention, there may be more than 20 steps or theremay be several steps repeated as a sequence (e.g., V1, V2, V3, V1, V2,V3, etc., where V1, V2 and V3 each are stepped voltages). In certainembodiments, the sequence of steps lasts between about 1 and about 10minutes. It's possible to have more or less time if desired, such as 8minutes. One example of the soft voltage initialization is shown in FIG.8A. As can be seen, a potential of 0.535V is applied for 2 minutes. Thena potential of 1.07V is applied for 2 minutes, after which the operatingpotential of 0.535V can be applied. Another example of the soft voltageinitialization is shown in FIG. 8B, where a series of 8 potential stepsare used to gradually step down from an initial potential of 1.07V tothe operating potential of 0.535V.

FIGS. 9A and 9B show the difference in response time with and without asoft initialization according to an embodiment of the present invention.FIG. 9A shows the signal response (iSig) of the sensor when the sensoris re-connected to the processor and sensor electronics after 2 hours ofdisconnection without any soft initialization. The start-up time (i.e.,the time to reach 90% of the expected response) is over 60 minutes long.FIG. 9B shows the signal response when the sensor is reconnected to theprocessor after 2 hours of disconnect using the soft initializationshown in FIG. 8A. In the case in FIG. 9B, the start up time is less than30 minutes, showing that the time can be greatly reduced even using asimple soft initialization method.

In alternate embodiments, other methods of hard voltage initializationfor a sensor may be used, including those discussed herein and in U.S.Pat. Nos. 5,320,725; 6,251,260 and U.S. Patent Application No.2005/0161346, the content of which are incorporated by reference.

A sample soft initialization is shown in FIG. 11. In the embodimentshown in FIG. 11, the sensor is disconnected for between 10 and 120minutes. When reconnected, the soft initialization starts with 2 minutesof 0.535V potential and then 2 minutes of 1.07V. The system them startsthe operating voltage of 0.535V. The particular embodiment in the figureis illustrative and different voltages and time periods could also beused. Some additional examples are shown in FIG. 14-17. FIG. 14 shows apotential step of 0.535V for 2 minutes, a potential step of −0.55V for 1minute, and a potential step of 1.07V for 2 minutes, which are thenfollowed by operating potential, e.g. at 0.535V. FIG. 15 shows a steppotential sequence of 0.535V for 1 minute, 1.07V for one minute, 0.535Vfor 1 minute, 0.85V for 1 minute, 0.535V for 1 minute, and 0.65V for 1minute, followed by operating potential, e.g. at 0.535V. FIG. 16 shows aseries of 8 potential steps that gradually steps down the voltage from1.07V to an operation potential of 0.535V. FIG. 17 shows a potentialstep of 0.535V for 2 minutes then 1.07V for 2 minutes followed byoperating voltage of 0.535V.

FIGS. 18 and 19 show boxplots of recovery time, which in those FIGs. isdefined as the time for the sensor to reach 90% of limiting current, forcertain soft initialization schemes. In FIG. 18, the schemes are 2minute rampdown with 10 steps (1801), 2 minutes at 0.535V and 2 minutesat 1.07V (1802), 2 minutes at 1.07V (1803), 30 seconds at 1.07V (1804),4 minute rampdown with 10 steps (1805), 5 minutes at 0.535V and 2minutes at 1.07V (1806), 60 seconds at 1.07V (1807), 4 minutes at 0.535Vand 1 minute at −0.55V and 2 minutes at 1.07V (1808), and a stepdownscheme (1809). In FIG. 19, a boxplot is shown of the sensor recoverytime based on the time of soft initialization, regardless of the softinitialization sequence used.

Continuing with FIG. 6, after the soft voltage initialization is appliedat step, stabilization is performed at step 605. The stabilization maybe performed by applying the operating voltage, for example 535 mV, fora certain amount of time, for example 10, 16, 20, or 26 minutes. At step610, the system determines whether the sensor is stable. If the sensoris not stable, the system performs additional stabilization until amaximum stabilization time or a maximum additional stabilization time isreached. In the embodiment shown in FIG. 6, the system determineswhether there has been stabilization for less than a maximumstabilization time, such as 30 minutes, at step 615. If not, thenadditional stabilization is performed at step 605. As withinitialization scheme 1, the additional stabilization can be for thesame amount of time as the original stabilization time or a differentamount of time. The system could be set up to have a series of smallstabilization times, such as 1 or 5 minutes followed by stabilizationchecks. In other embodiments, the stabilization check could be going onduring the stabilization period such that there is not a loop typestabilization process but instead a continuous stabilization withstabilization checks until stabilization is reached. If the sensor doesnot become stabilized after a maximum amount of time, the sensor is notuseable and should be removed and replaced. This is shown in FIG. 6 asstep 620. As shown in FIG. 6, if the sensor is stable, it is calibratedat step 625. Calibration may be performed using a blood glucose meter asdiscussed above or by other calibration methods. After calibration,sensing may begin at step 630.

A third scheme, initialization scheme 3, is shown in FIG. 7. In certainembodiments, it may be desired that sensors disconnected for a shortperiod of time not be subjected to a new initialization, as this limitedperiod of disconnection is insufficient for the sensor to need anyinitialization. A sample graph of initialization scheme 3 is shown inFIG. 12. In FIG. 12, the sensor has been disconnected for less than 10minutes, so the system merely applies the operating voltage of 0.535Vfor a stabilization period of 10 minutes before moving on to sensing atthe operating voltage. It is possible that this stability time be moreor less, depending on how long of a stabilization period is desired. Insome embodiments of the invention, as with initialization scheme 1 and2, there may be a stabilization check. As shown in FIG. 7, a sensor thathas fallen within the proper disconnection time range is stabilized atstep 701. The stabilization may be performed by applying the operatingvoltage, for example 535 mV, for a certain amount of time, for example10 minutes. Other time ranges could be employed, such as 16, 20 or 26minutes. At step 705, the system determines whether the sensor isstable. If the sensor is not stable, the system performs additionalstabilization until a maximum stabilization time or a maximum additionalstabilization time is reached. In the embodiment shown in FIG. 7, thesystem determines whether there has been stabilization for less than amaximum stabilization time, such as 30 minutes, at step 710. If not,then additional stabilization is performed at step 701. The additionalstabilization can be for the same amount of time as the originalstabilization time or a different amount of time. As with the otherinitialization schemes, the system could be set up to have a series ofsmall stabilization times, such as 1 or 5 minutes followed bystabilization checks. In other embodiments, the stabilization checkcould be going on during the stabilization period such that there is nota loop type stabilization process but instead a continuous stabilizationwith stabilization checks until a stabilization is reached. If thesensor does not become stabilized after a maximum amount of time, thesensor is not useable and should be removed and replaced. This is shownin FIG. 7 as step 715. In embodiments of the invention, no calibrationis required for sensors falling under this initialization scheme,although it is of course possible to recalibrate if desired. Thus,sensing begins at step 720.

In some embodiments of the invention, the methods can further compriseapplying a stabilization voltage to the sensor (e.g. a voltage designedto enhance sensor stability), for example after applying the selectedinitialization voltage, for a first stabilization time. The method mayfurther include determining whether the sensor is stable after applyingthe first stabilization voltage; and if the sensor is not stable,applying a second stabilization voltage to the sensor for a secondstabilization time. Example stabilization time periods include timesless than forty minutes, such as 10, 16, 20, and 26 minutes, or 30minutes. The second stabilization time may be the same or different thanthe first stabilization time.

Some embodiments of the invention comprise an improved method ofdetecting and/or facilitating sensor hydration. For detection ofhydration (e.g. the level or degree of implanted sensor hydration),voltage pulses are applied to the sensor immediately after insertion ofthe sensor and prior to sensor initialization using the sensorprocessor. The response (iSig) to this voltage pulse will be used fordetection of hydration. In one embodiment, a pre-initialization voltagepulse scheme for detecting hydration is a set of two alternatingvoltages, such as 0.0V and 0.2V. Other potential voltages could be used,for example alternating voltages between 0.0V and a second voltage ofbetween 0.1V and 0.535V. The sensor response (iSig) is recorded at ahigh sampling rate, such as every 1 second. The sensor is consideredhydrated when the response to the voltage pulse is above a certainhydration threshold, such as 100 nA. Once the sensor is consideredhydrated, the sensor undergoes initialization. If the threshold is notmet after a certain amount of time, for example 5 minutes or up to about20 minutes, the sensor will be considered not ready and will not goinitialization. Instead, more similar pulses, which may be the same asthe original pulses, are applied until the threshold is met. There maybe a maximum hydration time period after which the sensor will beconsidered non-functioning and should be removed and replaced by a newsensor. FIG. 13 is an example graph showing the sensor response duringhydration detection according to the embodiment shown above.

Some embodiments of the invention include detecting hydration of thesensor prior to applying the selected initiation protocol, whereindetecting hydration includes applying a series of hydration pulses(voltages selected to detect or facilitate sensor hydration) to thesensor for a first hydration time; recording the current response of thesensor during application of the series of hydration pulses; andcomparing the current response to a predetermined hydration threshold.Application of the series of hydration pulses may be terminated if thecurrent response reaches or exceeds the predetermined hydrationthreshold. Detecting hydration may further include applying a secondseries of hydration pulses to the sensor for a second hydration time ifthe current response does not reach the predetermined hydrationthreshold during the first predetermined hydration time. Thepredetermined hydration threshold may be 100 nA or 50 nA, for example.Example hydration pulses may be a series of 0 V and 2 V pulses, forexample for 20 seconds or 2 minutes each.

The hydration detection described above may be used in combination withother methods, for the same or different sensors. For example, in somemethods where more than one sensor is used, the hydration detectiondescribed above may be used with respect to one sensor and a differenthydration detection may be used with respect to the other sensor. Thesensors should become hydrated at roughly the same time, so this wouldserve as a check to make sure that they hydration detection methods areall correlating properly.

One example of another method to determine hydration includes using aprocessor that detects whether a sensor is sufficiently hydrated foranalyte detection comprising a computer usable media including at leastone computer program embedded therein that is capable of calculating animpedance value; and comparing the impedance value against a thresholdto determine if the sensor is sufficiently hydrated for analytedetection. In related methods, detecting whether a sensor issufficiently hydrated for analyte detection includes comprisingcalculating an open circuit potential value between at least twoelectrodes of the sensor and comparing the open circuit potential valueagainst a threshold to determine if the sensor sufficiently hydrated foranalyte detection. Typically, the open circuit potential value is theimpedance value (and optionally this value is an approximation of a sumof polarization resistance and solution resistance). Optionally, theopen circuit potential value is compared against an another threshold todetermine if the sensor sufficiently hydrated for analyte detection.This can solve problems that occur when a user attempts to initialize asensor that is not fully hydrated (e.g. compromising the accuracy and/orlifetime of the sensor).

Some embodiments of the invention include a fuse element that can betriggered after a predetermined period of time or event so as tointerrupt a flow of electrical current within the apparatus (i.e. so asto disable the sensor), as disclosed in U.S. patent application Ser. No.12/184,046 (filed Jul. 31, 2008), which is herein incorporated byreference.

In some embodiments of the invention, a processor is capable ofcomparing a first signal received from a working electrode in responseto a first working potential with a second signal received from aworking electrode in response to a second working potential, wherein thecomparison of the first and second signals at the first and secondworking potentials can be used to identify a signal generated by aninterfering compound. These methods are further discussed in U.S.application Ser. No. 12/184,046 (filed Jul. 31, 2008), which is hereinincorporated by reference.

Certain sensor embodiments switch between a high potential to a lowpotential (e.g. with a frequency of less than 3, 2 or 1 seconds). Insuch embodiments, a sensor may not discharge, with for example sensorelements acting as a sort of capacitor. In this context, someembodiments of the invention can include a circuit discharge elementthat facilitates sensor circuit discharge (e.g. if discharge is notsufficient to reach a specific potential such as 535 millivolts). Avariety of such circuit discharge elements known in the art can beadapted for use with sensor embodiments of the invention (see, e.g. U.S.Pat. Nos. 4,114,627; 4,373,531; 4,858,610; 4,991,583; and 5,170,806,5,486,201, 6,661,275 and U.S. Patent Application No. 20060195148).Optionally for example, a sensor charge can be removed by connecting itthrough a discharging switch element, and optionally a dischargingresistor element.

Sensors of the invention can also be incorporated in to a wide varietyof medical systems known in the art. Sensors of the invention can beused, for example, in a closed loop infusion systems designed to controlthe rate that medication is infused into the body of a user. Such aclosed loop infusion system can include a sensor and an associated meterwhich generates an input to a controller which in turn operates adelivery system (e.g. one that calculates a dose to be delivered by amedication infusion pump). In such contexts, the meter associated withthe sensor may also transmit commands to, and be used to remotelycontrol, the delivery system. Typically, the sensor is a subcutaneoussensor in contact with interstitial fluid to monitor the glucoseconcentration in the body of the user, and the liquid infused by thedelivery system into the body of the user includes insulin. Illustrativesystems are disclosed for example in U.S. Pat. Nos. 6,558,351 and6,551,276; PCT Application Nos. US99/21703 and US99/22993; as well as WO2004/008956 and WO 2004/009161, all of which are incorporated herein byreference.

A number of articles, U.S. patents and patent application describe thestate of the art with the common methods and materials disclosed hereinand further describe various elements (and methods for theirmanufacture) that can be used in the sensor designs disclosed herein.These include for example, U.S. Pat. Nos. 6,413,393; 6,368,274;5,786,439; 5,777,060; 5,391,250; 5,390,671; 5,165,407, 4,890,620,5,390,671, 5,390,691, 5,391,250, 5,482,473, 5,299,571, 5,568,806; UnitedStates Patent Application 20020090738; as well as PCT InternationalPublication Numbers WO 01/58348, WO 03/034902, WO 03/035117, WO03/035891, WO 03/023388, WO 03/022128, WO 03/022352, WO 03/023708, WO03/036255, WO03/036310 and WO 03/074107, the contents of each of whichare incorporated herein by reference.

Typical sensors for monitoring glucose concentration of diabetics arefurther described in Shichiri, et al.: “In Vivo Characteristics ofNeedle-Type Glucose Sensor-Measurements of Subcutaneous GlucoseConcentrations in Human Volunteers,” Horm. Metab. Res., Suppl. Ser.20:17-20 (1988); Bruckel, et al.: “In Vivo Measurement of SubcutaneousGlucose Concentrations with an Enzymatic Glucose Sensor and a WickMethod,” Klin. Wochenschr. 67:491-495 (1989); and Pickup, et al.: “InVivo Molecular Sensing in Diabetes Mellitus: An Implantable GlucoseSensor with Direct Electron Transfer,” Diabetologia 32:213-217 (1989).Other sensors are described in, for example Reach, et al., in ADVANCESIN IMPLANTABLE DEVICES, A. Turner (ed.), JAI Press, London, Chap. 1,(1993), incorporated herein by reference.

Various publication citations are referenced throughout thespecification. In addition, certain text from related art is reproducedherein to more clearly delineate the various embodiments of theinvention. The disclosures of all citations in the specification areexpressly incorporated herein by reference.

What is claimed is:
 1. A method of initializing an analyte sensor,comprising: determining a disconnection time, wherein the disconnectiontime is the amount of time the sensor has been disconnected from sensorelectronics, selecting an initialization protocol based on thedisconnection time, the initialization protocol selected from the groupconsisting of: a first initialization scheme comprising a first seriesof voltage pulses and a second initialization scheme comprising a secondseries of voltage pulses, wherein the first initialization scheme isselected if the disconnection time falls within a first time range andthe second initialization scheme is selected if the disconnection timefalls within a second time range; and applying the selectedinitialization protocol to the sensor.
 2. The method of claim 1, whereinthe initialization protocol is selected from the group furtherconsisting of a third initialization scheme comprising the applicationof no voltage to the sensor, wherein the third initialization scheme isselected if the disconnection time is less than the first time range andthe second time range.
 3. The method of claim 1, further comprisingapplying a stabilization voltage to the sensor, after applying theselected initialization voltage, for a first stabilization time.
 4. Themethod of claim 3, further comprising: determining whether the sensor isstable after applying the first stabilization voltage; and if the sensoris not stable, applying a second stabilization voltage to the sensor fora second stabilization time.
 5. The method of claim 4, furthercomprising calibrating the sensor if the sensor is stable.
 6. The methodof claim 5, wherein calibrating the sensor includes measuring bloodglucose using a blood glucose meter.
 7. The method of claim 5, whereincalibration of the sensor is only performed if the disconnection timefalls within the first or second time range.
 8. The method of claim 4,wherein if the sensor is not stable after a predetermined maximumstabilization time, the initialization protocol is ended so that a newsensor may be connected to the sensor electronics.
 9. The method ofclaim 8, wherein the maximum stabilization time is 30 minutes.
 10. Themethod of claim 1, wherein the determining a disconnection time includesmeasuring the current output of the sensor and comparing the measuredcurrent output to a disconnection threshold value.
 11. The method ofclaim 10, wherein the determining a disconnection time further includescomparing the current output to a reconnection threshold value.
 12. Themethod of claim 10, wherein the disconnection threshold value is 0.6 nA.13. The method of claim 1, wherein the first time range is adisconnection time of greater than 120 minutes.
 14. The method of claim1, wherein the second time range is 10 minutes to 120 minutes.
 15. Themethod of claim 1, wherein the second initialization scheme comprisesapplication of at least two voltages to the sensor for a predeterminedsecond initialization time.
 16. The method of claim 15, wherein the atleast two voltages are a series of stepped down voltages.
 17. The methodof claim 15, wherein the predetermined second initialization time isless than 30 minutes.
 18. The method of claim 1, further comprisingdetecting hydration of the sensor prior to applying the selectedinitialization protocol, wherein detecting hydration includes: applyinga series of hydration pulses to the sensor for a first hydration time;recording the current response of the sensor during application of theseries of hydration pulses; and comparing the current response to apredetermined hydration threshold.
 19. The method of claim 18, whereinapplication of the series of hydration pulses is terminated if thecurrent response reaches or exceeds the predetermined hydrationthreshold.
 20. The method of claim 18, wherein the detecting hydrationfurther includes applying a second series of hydration pulses to thesensor for a second hydration time if the current response does notreach the predetermined hydration threshold during the firstpredetermined hydration time.
 21. The method of claim 18, wherein thepredetermined hydration threshold is 100 nA.
 22. A method ofinitializing an analyte sensor, comprising: determining a disconnectiontime, wherein the disconnection time is the amount of time a sensor hasbeen disconnected from sensor electronics, selecting an initializationprotocol based on the disconnection time, the initialization protocolselected from the group consisting of: a first initialization schemecomprising a first series of voltage pulses, a second initializationscheme comprising a second series of voltage pulses, and a thirdinitialization scheme comprising the application of no voltage to thesensor, wherein the first initialization scheme is selected if thedisconnection time falls within a first time range, the secondinitialization scheme is selected if the disconnection time falls withina second time range, and the third initialization scheme is selected ifthe disconnection time is less than the first time range and the secondtime range; applying the selected initialization protocol to the sensor;and applying a stabilization voltage to the sensor for a firststabilization time.